Fire Suppression - Natural Range of Variation for Yellow Pine and Mixed-Conifer Forests in the

The evolution of many plant species has been greatly influenced by their long-term relationships with fire (Bond and van Wilgen 1996, Keeley et al. 2012, Sugihara et al. 2006). Human-caused changes to natural fire regimes can have significant impacts on the diversity and composition of native plant and animal communities (Hobbs and Huenneke 1992, Mutch 1970). Because of the high ecological and evolutionary importance of frequent fire to assessment area YPMC forests, it has Broadleaf woodland

been argued that the exclusion of fire from most of the assessment area for the last century is one of the most significant human-caused ecological disturbances cur -rently in play (Barbour et al. 1993, Fites-Kaufman et al. 2007, Sugihara et al. 2006).

The scientific and management literature is overflowing with assessments of, and references to, the deleterious effects of fire exclusion on assessment area ecosystems (summaries in Agee 1993; Barbour et al. 1993, 2007; Erman and SNEP Team 1996, Keeley et al. 2012; Sugihara et al. 2006, and others.). These include altered species composition and dominance patterns, increased fuels and forest density, impacts to soils and hydrological cycles and carbon sequestration, loss of important wildlife habitat, increased fire intensity and severity, decreased human safety, threats to infrastructure, and so on.

Cermak (2005) provided a detailed consideration of the development of the fire control organization and policies in California. The desire to control fire came largely as a response to the destructive burning practices of early settlers, and also the belief that frequent fires were destroying timber and reducing the capacity of the forest to regenerate. Controversies surrounding the use of “light burning” to reduce forest fuels and protect old growth developed in the 1910s and 1920s, and again in the 1950s, but in both cases proponents of fire exclusion prevailed. In 1910, Region 5 (Pacific Southwest Region) Regional Forester Coert DuBois directed his forest supervisors that fire control was the top management priority in the Forest Service’s Pacific Southwest Region. He followed this with the 1914 publication of Systematic Fire Protection in the California Forests, which Cermak (2005) called the “most influential single document in U.S. fire control history.” It set fire control standards (forest fires were to be controlled before they reached 10 ac [4 ha]), and it described the outlines for a formal fire control organization and the processes for coordinated fire planning. In 1919, Region 5 directed forest supervisors to suppress all fires, even on neighboring private land. In 1924, the California Board of Forestry endorsed “fire exclusion” from forest lands as state policy. A policy of overnight fire control was discussed at a national Forest Service meeting in 1935, and emerged as the famous “10 a.m.” rule in May of that year (whereby Forest Service units were expected to have fire starts controlled by 10 a.m. the day after discovery).

Federal land managers were already actively working to extinguish fires when the first forest reserves were established in California at the end of the 19^th century, but the lack of training, coordination, planning, and technology meant that their ability to stop large fires was very limited (Cermak 2005). The first trained fire crews were established in the late 1920s, and the adoption of more modern tech -niques and technologies gradually led to increasing success in fire suppression.

Consultation of the California Fire Perimeters database (see footnote 4) shows a

The exclusion of fire

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strong drop in fire frequency and annual burned area in the 1930s and 1940s. This was helped by a series of wet years in the late 1930s, but by the end of the 1940s a number of innovations had markedly improved firefighting success, including the deployment of tanker trucks and bulldozers, the institution of “hotshot” fire crews and “smokejumpers,” and the expanded use of planes and helicopters in patrols and aerial water drops (Cermak 2005).

Patterns in fire frequency show remarkable success in fire control through most of the rest of the 20^th century (the Forest Service succeeds in extinguishing 98 percent of all ignitions before they reach 300 ac [120 ha;] [Calkin et al. 2005]), but beginning in the 1980s, the area of forest burned began to climb. By the 1990s, 10-year running averages for annual burned area and average fire size were at their highest points since formal recordkeeping began in 1908, and the upward trend continues (Calkin et al. 2005; see figures in Miller et al. 2009b). Since 1910, 15 fires have exceeded ^~20 000 ha (~50,000 ac) in size in the assessment area, but 14 of these have occurred since the late 1970s, and 12 since 1995.

Part of the trend in area burned and fire size is due to changed federal fire management policies. In the face of research and management reviews showing the detrimental ecological effects of fire exclusion on Western forest ecosystems (e.g., Biswell 1961, Leopold et al. 1963), the National Park Service began permit-ting prescribed fires in California in the late 1960s and early 1970s, and allowed some lightning ignitions to burn under prescribed conditions (van Wagtendonk et al 2002). At the same time, Forest Service wilderness areas experimented with management, rather than suppression, of naturally ignited fires (Stephens and Ruth 2005). The Forest Service changed its policy from strict fire control to fire manage -ment in 1974, and formally abandoned the 10 a.m. rule in 1978 (Pyne 1982). By limiting direct attack on difficult fires, and taking greater advantage of topography, natural barriers, and weather to “indirectly” control fires, fire management agencies themselves have played a role in the growth of large fires since the late 1970s.

Nonetheless, the evidence is overwhelming that accumulated fuels and changes in forest structure resulting from a century of fire exclusion have led to major ecosystem changes in forest types that experienced frequent, primarily low-severity fires before Euro-American settlement (e.g., Agee 1993; Barbour et al. 1993, 2007;

Erman and SNEP Team 1996; Leopold et al. 1963; Parsons and DeBenedetti 1979;

Steel et al. 2015; Sugihara et al. 2006; etc). In interaction with climate warming, these forest changes are now resulting in larger and more severe fires throughout the YPMC forest belt, not only in the assessment area but across the southwestern United States (Dillon et al. 2011, Mallek et al. 2013, Miller and Safford 2012, Miller et al. 2009b, Skinner and Chang 1996). In summary, fire suppression is a major Since 1910, 15 fires

disturbance factor in assessment area YPMC forests, both in its direct modifica -tion of ecosystem composi-tion, structure, and func-tion, and in its contribu-tion to increased forest fuels amounts and continuity, which are leading to deleterious effects when forest fires escape control. Nearly every other section in this chapter contains additional information pertaining to the negative ecological effects of fire suppression on YPMC forests.

Grazing

American Indian inhabitants of the assessment area did not herd animals, and livestock grazing occurred only after Euro-American settlement. A short sum -mary of the grazing history of YPMC forests is offered here to provide context to current conditions and to the early observations that Euro-Americans made of the assessment area. For additional grazing-related information, see “Grass and forbs”

on page 153.

Appreciable livestock grazing began in assessment area YPMC forests after the arrival of Euro-American settlers after 1849. By the 1860s, valley and foothill ranchers were using public lands in the Sierra Nevada on a seasonal basis to graze their herds of cattle (Dasmann 1965, Jackson et al. 1982, Pease 1965). Sheep grazing was also practiced in much of the Sierra Nevada after about 1860 (McKelvey and Johnston 1992). The herding habits, huge numbers, and more general diet of sheep caused major effects on Sierra Nevada ecosystems, especially riparian areas and meadows, and probably affected fire regimes as well by reducing fine fuels. Leiberg (1902) viewed grazing, especially by sheep, as a “destructive agent to the forest by preventing reforestation.” Muir (1894) referred to sheep as “hoofed locusts.”

Sudworth (1900) militated for stricter control of sheep grazing. Conflicts developed between cattle ranchers and shepherds; and public concern with the effects of graz -ing, particularly by sheep, was one of the factors leading to the designation of the forest reserves in the 1890s and early 1900s. Shepherds and cattlemen also often set fire to the forest in the late summer or fall to clear the forest understory and osten-sibly to improve forage; in some cases these fires caused major damage to YPMC and red fir forests, mostly where previous logging had increased surface fuel loads (Cermak 2005, Greeley 1907, Jackson et al. 1982, Leiberg 1902, Sudworth 1900).

Vankat and Major (1978) noted that livestock grazing, especially by sheep, had affected most of Sequoia National Park. However, their references to specific records of overgrazing refer almost exclusively to montane meadows and high-elevation forests, and they do not list grazing as a major change agent for YPMC forests. Sheep grazing in the park ceased in the early 20^th century, so there has been nearly a century for park ecosystems to recover.

GENERAL TECHNICAL REPORT PSW-GTR-256

Brewer’s memoirs from 1861 to 1864 (Brewer 1930) referred to the scarcity of good grass cover in the Sierra Nevada, and lush areas of grass were highlighted where they occurred. Brewer’s team traveled on horseback, so they were reliant on the avail-ability of forage. Brewer’s time in the Sierra Nevada predated heavy cattle or sheep grazing, although he mentioned grazing in his memoirs. Sudworth’s team also relied on pasturage, and Sudworth (1900) noted that unfenced forest land supported very

“short forage” and expressed the opinion that sheep grazing had decimated herbaceous and grass cover in much of the central and southern Sierra Nevada, basing his state -ment on the “study of long-protected forest land in the same region” and conversations with older settlers. Most of Sudworth’s unpublished notes refer to higher elevation locations, however, not mixed conifer. Leiberg (1902) also primarily referred to higher elevations (red fir, principally) when discussing the deleterious effects of grazing.

The period between 1894 and 1904 was extremely dry across southern Cali -fornia (but not as catastrophically dry in northern Cali-fornia, except between 1897 and 1899) (USDI 1951); most of the oft-cited observations of deleterious impacts of heavy grazing on Sierra Nevada ecosystems are from this period (e.g., those cited in McKelvey and Johnston 1992). Note also that the years in which Sudworth (1900) and Leiberg (1902) conducted their field studies coincided with the third longest recorded period of profound drought in California (as measured by the Palmer Drought Severity Index—PDSI), which included the 2^nd driest year on record (1898), and the 3^rd driest 2-year span (1898–1899) (NOAA National Climate Center data for 1895 to 2015: https://www.ncdc.noaa.gov/cdo-web/search?datasetid=GHCND). The extreme dryness of the soil and depleted herbaceous cover noted by observers during this period was ascribed by many of them entirely to sheep grazing, but the extreme climatic conditions certainly played a major role. Old settler’s memories were of times before significant sheep grazing but also of much more abundant rainfall.

Whatever the case, heavy grazing in much of the assessment area clearly reduced understory cover and affected soil in parts of the assessment area for many decades. It probably also reduced fire frequency in some parts of assessment area YPMC forests by reducing the amount of fine fuel. Swetnam and Baisan (2003) noted that many Sierra Nevada fire histories show a virtual absence of fire after the 1850s, which they attributed to the introduction of large herds of sheep into the range after the 1859–1860 drought. A soon to be published fire history study from the southern Modoc Plateau (Adin Pass area) shows a very early cessation of fires that coincides closely with the introduction of cattle to the study area, and other studies showing very low local FRIs (<8 years) also suggest that herbaceous fine fuels would have been necessary to support such high fire frequencies (see footnote 8). These studies are in northern and northeastern assessment area forests dominated by ponderosa pine, and may follow the model suggested for the Southwestern United

States (Arizona and New Mexico) by Swetnam and Betancourt (1998), where fire regimes in ponderosa pine forest were hypothesized to respond strongly to herba -ceous fuel production, while fire regimes in mixed-conifer forests were suggested to be driven primarily by woody fuels. Note, however, that the general lack of summer precipitation in the assessment area results in much less grass cover on average than in yellow pine forests in the Southwestern United States, which receives much sum -mertime rainfall; see “Forest understory and nonforest vegetation” on page 146.

Given that most heavy (especially sheep) grazing ceased before World War I, one question is to what extent the effects of this disturbance have lasted over the ensuing century. Grasses are extremely resilient to disturbance, and their seeds are very easily dispersed. A further issue is that the institution of fire suppression and the cessation of heavy grazing happened at about the same time. This complicates our ability to discern the independent effects of the two disturbances. It also means that forests were densifying (and reducing understory light availability) just as understory plant communities were being freed from decades of heavy pasturage.

This probably stalled understory recovery and possibly led to different patterns of succession than would have occurred had fire not been suppressed.

Insects and Disease

⁹

Background information—

Table 5 lists major insects and diseases found in tree species of assessment-area YPMC forests. A more complete listing and description of injurious insects and diseases can be found at http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev3_046410.pdf.

Noteworthy increases in ponderosa pine, Jeffrey pine, and sugar pine mortality in the Sierra Nevada range can usually be attributed to moisture stress, high tree density, and elevated bark beetle activity (Young et al. 2017). The primary bark beetles associated with ponderosa pine mortality are western pine beetle ( Dendroc-tonus brevicomis) and mountain pine beetle (DendrocDendroc-tonus ponderosae). Mountain pine beetle also kills sugar pine. Jeffrey pine beetle is the primary killer of Jeffrey pine. Dwarf mistletoe (Arceuthobium M. Bieb.) and Heterobasidion root disease cause additional stress on host trees; the spread of Heterobasidion is abetted by log -ging when cut stump faces are not treated with borax (Slaughter and Rizzo 1999).

Black stain root disease is scattered throughout the northern Sierra Nevada range and can be found in ponderosa and Jeffrey pine. White pine blister rust has been devastating to sugar pine since the disease entered northern California around 1930.

9 “Insects and Disease” text primarily by Sheri Smith, USDA, Forest Service, Pacific Southwest Region regional entomologist, Lassen National Forest, 2550 Riverside Drive, Susanville, CA 96130.

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White fir mortality throughout the Sierra Nevada is typically attributed to fir engraver beetle, moisture stress, and Heterobasidion root disease. High tree density and true fir dwarf mistletoe (Arceuthobium abietinum (Engelm.) Hawksw. &

Wiens) also contribute toward decline in some areas. Douglas-fir tussock moth readily defoliates white fir in the Sierra Nevada. Population cycles trend upward every 7 to 10 years, and significant levels of tree mortality have been recorded during past outbreaks.

Insects rarely kill incense cedar. Bark beetles that attack incense cedar are not considered aggressive tree killers; however, when combined with drought stress, they can cause mortality. During drought periods in some areas of the Sierra Nevada range, small incense cedars are the first trees to decline and die. Heteroba-sidion root disease and true mistletoe also weaken incense cedar.

Douglas-fir in the Sierra Nevada can be heavily affected by insects or diseases typical of more northerly latitudes, but their incidence in the Sierra Nevada is reduced. However, Douglas-fir beetle, flatheaded fir borer, and black stain root disease can be found in some Douglas-fir stands. Both insects are capable of killing trees, particularly drought-stressed ones. The detected incidence of black stain root disease in Douglas-fir in the Sierra Nevada is low.

Several insects and diseases can be found on native oaks. Typically the extent or severity of their effects are not widespread or protracted. Foliar injury can result Table 5—Major insects and diseases found in tree species of assessment-area yellow pine and mixed-conifer forests^a

Host tree^b

Agent Pp Pj Pl Ac Cd Pm Qk Sg

Heterobasidion root disease, Heterobasidion spp. x x x x x x x

Black stain root disease, Leptographium wageneri x x x x

Armillaria root disease, Armillaria spp. x x x x x x x x

Dwarf mistletoe, Arceuthobium spp. x x x x x

White pine blister rust, Cronartium ribicola x

Western pine beetle, Dendroctonus brevicomis x

Jeffrey pine beetle, Dendroctonus jeffreyi x

Mountain pine beetle, Dendroctonus ponderosae x x

Fir engraver beetle, Scolytus ventralis x

Douglas-fir beetle, Dendroctonus pseudotsugae x

Douglas-fir tussock moth, Orgyia pseudotsugata x

Flatheaded fir borer, Melanophila drummondi x x

a Table courtesy of Sherri Smith, USDA Forest Service Pacific Southwest Region regional entomologist.

b Host species and codes: Pp = ponderosa pine, Pinus ponderosa; Pj = Jeffrey pine, P. jeffreyi; Pl = sugar pine, P. lambertiana; AC = white fir, Abies concolor; Cd = incense cedar, Calocedrus decurrens; Pm = Douglas-fir, Pseudotsuga menziesii; Qk = California black oak, Quercus kelloggii; Sg = giant sequoia, Sequoiadendron giganteum.

from a variety of diseases, insects, and mites. Wood-boring beetles are usually restricted to dead or dying branches, although the recent emergence of gold spotted oak borer in southern California is a worrying sign, as it readily kills adult black oak, canyon live oak, and coast live oak. Damage by these agents is normally sec -ondary in nature, rather than the primary cause of branch or tree decline. Armillaria root disease and true mistletoe can commonly be found on oaks.

Uprooting and stem breakage of giant sequoia is not uncommon and can be a problem along roads and in recreation areas. Heterobasidion root disease is some-times found infecting the roots of fallen trees. Tree killing of giant sequoias by insects or diseases is rare.

NRV and comparison to current—We have little information on insect or disease occurrence in presettlement YPMC forests in the assessment area. Based on insect and forest ecology, however, some inferences can be made about probable changes over time. Fettig (2012) provided a list of the bark beetle species that cause “signifi -cant” mortality in the assessment area. Within YPMC forests, most research has been done on the beetles affecting yellow pine species, especially beetles from the genus Dendroctonus, as they can have major impacts on mortality rates in commer -cially important stands of trees.

It has been understood for some time that tree stand densities have a strong relationship to bark beetle-induced mortality. Higher density stands increase competition for resources (especially water and light) and reduce tree vigor, which makes individual trees less able to withstand insect attack. Various studies dem -onstrate that lower density YPMC stands are much less susceptible to bark beetle attack and subsequent mortality (Fettig et al. 2007, Young et al. 2017).

In the current absence of frequent understory fire, bark beetles have become one of the principal agents of tree mortality in the assessment area (Fettig 2012, Manley et al. 2000). Under reference conditions, frequent fire would have interacted with insects and disease, as well as abiotic and biotic site conditions, to drive stand structure (Bonnicksen and Stone 1982, North et al. 2012b). Much more open and heterogeneous forest structure resulted, and—based on the strongly inverse stand density versus bark beetle relationship—we can infer that bark beetle-caused mortality was probably lower than under current conditions.

Evidence from comparisons between contemporary forests in the assessment area and reference sites in Baja California support this inference. The Lake Tahoe Water -shed Assessment compared modern disease and insect incidence in old-growth forest versus mid-seral forests in the Lake Tahoe basin and also versus old-growth forests in the SSPM (Manley et al. 2000). Mid-seral forests in the Lake Tahoe basin generally

Dans le document Natural Range of Variation for Yellow Pine and Mixed-Conifer Forests in the Sierra Nevada, Southern Cascades, and Modoc and Inyo National Forests, California, USA (Page 74-86)